The Collaborative International Dictionary of English v.0.48:

Wellington boot \Wel"ling*ton boot\ [After the Duke of
   Wellington.]
   A riding boot for men, the front of which came above the
   knee; also, a similar shorter boot worn under the trousers.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Wellington boot
    n 1: (19th century) a man's high tasseled boot [syn: Hessian
         boot, hessian, jackboot, Wellington, Wellington
         boot]